It is sometimes desired to reinforce various rubber compositions with short, discontinuous fibers for various purposes. For example, see U.S. Pat. No. 4,871,004.
Use of only a very minor amount (e.g. from 4 to 8 phr) of short fibers in a rubber composition formulation typically substantially increases the viscosity (e.g. Mooney viscosity) of the uncured rubber composition to an extent which may make the rubber composition more difficult to mix in an internal rubber mixer and otherwise process. It is believed that such phenomenon is well known to those having skill in such art.
Where it is desired to provide a durable cured rubber composition as a component in various articles such as, for example, a component of an industrial belt or as an insert in a tire sidewall to stiffen the sidewall, it might seem logical to contemplate the use short fibers as reinforcement for such rubber composition.
Silica rich rubber compositions conventionally shrink somewhat upon sulfur vulcanization. It has been previously suggested, in the case of silica reinforced tire treads which contain at least 30 phr of silica which is accompanied by a silica coupling agent (see U.S. Pat. No. 5,852,097), to add about 2 to about 10 phr of short fibers to alleviate, or at least reduce such shrinkage. Apparently, the chemical reaction between the silica coupling agent and silica is necessarily expected to continue in order that the silica in the silica rich rubber composition fulfills its use as a reinforcing filler which is coupled to the elastomer(s) of the tire tread rubber composition.
Significantly, while it has been observed herein that addition of small amounts of short fiber reinforcement in a silica-free, carbon black reinforced rubber composition does increase its uncured viscosity so that processing of the uncured rubber has to be taken into consideration, one or more physical properties of the cured rubber composition may be degraded.
For example, it has heretofore been observed that although a 300 percent modulus of such a carbon black reinforced cured rubber composition may increase upon addition of a short fiber, the ultimate tensile strength of the cured rubber composition has been reduced. The reduction in tensile strength of a rubber composition may be a serious matter where such cured rubber composition is relied for its durability during service conditions for the rubber composition.
While such phenomenon may not be fully understood, it is believed that it is due, at least in part, to a very small degree of relative mobility of the short fibers within the cured rubber composition as compared to particulate carbon black reinforcement during service conditions during which the rubber composition may be subject to extensive dynamic flexing even though such flexing may only involve relative short extensions, or deformations, of the rubber composition itself.
Accordingly, it has been contemplated herein to add an anchoring agent to a substantially silica-free, carbon black reinforced rubber composition to provide some degree of anchoring at least a portion of short fibers contained in a rubber composition and thereby prevent, or at least retard, their small relative movement within the rubber composition during service conditions.
While various anchoring agents may be considered, use of coupling agents sometime used for coupling amorphous silica to various diene-based rubber compositions are considered herein. While such silica coupling agents typically chemically react with the surface of the silica to couple the silica to an elastomer by having a moiety which is chemically reactive with hydroxyl groups on the surface of the silica (e.g. silanol groups) and another moiety which is interactive with a diene-based elastomer of the rubber composition itself.
In the description of this invention, the term "phr" means parts by weight of a specified ingredient per 100 parts by weight of elastomer.
Also, in the description of this invention, the terms "rubber" and "elastomer" are used interchangeably, unless otherwise noted.
The terms "vulcanized" and "cured" are also used interchangeably unless otherwise noted.